Microwave Microplasma Research Articles

Particle-in-cell simulations of the ionization process in microwave argon microplasmas

The importance of microwave device reliability and performance for microscale devices motivates a more fundamental understanding of breakdown mechanisms in this regime. Microwave breakdown theories predict breakdown when electron production balances electron loss. Electron production depends strongly on the ionization rate νi; however, previous studies either used the measured νi in macroscale gaps or the empirical formula for DC voltage, inaccurately predicting νi in microscale gaps. Alternatively, this work characterizes νi in microwave microplasmas by using particle-in-cell simulations. We calculated νi in argon gas at atmospheric pressure for 2–10 μm gaps under AC fields ranging from 1 to 1000 GHz. The behavior of νi may be separated into two regimes by defining a critical frequency fcr that depends on the amplitude of the applied voltage, gap distance, and pressure. For frequency f<fcr, the electrodes collect the electrons during each cycle and the electron number oscillates with the electric field, causing νi/f to roughly scale with the reduced effective field Eeff/p. For f>fcr, the phase-space plots indicate that the electrons are confined inside the gap, causing the electron number to grow exponentially and vi/p to become a function of Eeff/p. These results elucidate the ionization mechanism for AC fields at microscale gap distances and may be incorporated into field emission-driven microwave breakdown theories to improve their predictive capability.

SOMAFOAM: An OpenFOAM based solver for continuum simulations of low-temperature plasmas

We report the development of SOMAFOAM, a finite volume framework for performing continuum simulations of low-temperature plasmas. The primary goal of this work is to discuss the features of SOMAFOAM along with representative results provided as examples for a range of operating conditions and geometries. This includes plasma and plasma–dielectric systems operating in direct current, radio frequency, and microwave regimes from pressures as low as 100 mTorr to atmospheric pressure. The code has several useful features including the ability to run massively parallel simulations using arbitrary geometries, structured/unstructured meshes, choice of models such as drift–diffusion/full-momentum at runtime, and species-dependent timesteps to name a few. The verification/validation studies presented include comparison with previously published continuum simulations (low-pressure direct current and radio frequency plasma), with experiments (Gaseous Electronics Conference Reference Cell and microwave microplasma ignited in a split ring resonator), and previously published kinetic simulations (low-pressure radio frequency plasma). Other examples provided include a direct current atmospheric pressure microplasma bounded by dielectric sidewalls and a helium–nitrogen plasma ignited using a needle electrode facing a dielectric. The performance of the code is also discussed with serial and distributed memory parallel runs demonstrated up to 512 cores. The design and implementation of the code in a modular object-oriented framework allows for easy extension and seamless coupling with other codes and can be expected to play an important role in both academia and industry.

A kinetic study of electron heating and plasma dynamics in microwave microplasmas

Microwave microplasmas ignited in argon are studied using a one-dimensional particle-in-cell with Monte Carlo collision (PIC-MCC) approach. One-dimensional PIC-MCC simulations are performed at specified input power densities to determine the influence of the applied frequency (ranging from 1 to 320 GHz), pressure, and total deposited power on the plasma dynamics. The frequency response study performed at a fixed input power density shows the presence of off-axis peaks in the electron number density profile at intermediate frequencies. These peaks are attributed to the interplay between the production of hot electrons by the oscillating sheath and their inability to diffuse sufficiently at higher operating pressures, thereby resulting in enhanced ionization at off-axis locations. This is confirmed by the pressure dependence study which shows that the electron number density peaks at the mid-point when the microplasma is ignited at lower pressures. As the excitation frequency is increased further, the sheath oscillation heating decreases and eventually vanishes, thereby requiring the bulk plasma to couple power to the electrons which in turn leads to an increase in electron temperature in the plasma bulk and the electron number density peak appearing at the mid-point. When the power coupled to the microplasma is decreased, the sheath oscillation at a given frequency decreases, thereby leading to higher contribution from heating in the bulk plasma which leads to the disappearance of off-axis peaks even at intermediate frequencies. The microplasma dynamics at all conditions considered in this work demonstrate the interplay between the electron momentum transfer collision frequency, the angular excitation frequency, and the plasma frequency.

NO Abatement Using Microwave Micro Plasma Generated With Granular Activated Carbon

Abatement of NO using microwave micro-plasma is presented in this paper. The micro-plasma is generated using granular activated carbon (GAC) particles of size (2–3 mm) in loosely fluidized bed in a microwave cavity operated at 2.45 GHz. A single-mode microwave cavity reactor (SMMCR) was constructed and microwave was injected through another slotted single-mode waveguide in a sandwiched manner. COMSOL Multiphysics software was used to investigate the microwave electric field and the power density within the SMMCR. Gas mixture of air and 500 ppm NO (in N2) at the flow rate of 2 l/min was passed through a quartz tube centered within the SMMCR, while the supplied microwave power was very low 10–80 W and corresponding NO reduction was greater than 98%. The mass of GAC used for generating the plasma was 5g. The efficiency of NO reduction is found to be 24.84 g(NO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{2}}$</tex-math></inline-formula> )/kWh. When air is mixed with NO (in N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{\text{2}}$</tex-math></inline-formula> ), the efficiency of NOx reduction achieved vary greatly with respect to the supplied microwave energy, and behavior has become complex and is not predictable, which needs further investigation. A gas analyzer (testo 350) was used to measure the gas (NO, NO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{\text{2}}$</tex-math></inline-formula> , CO, and O <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{2}}$</tex-math></inline-formula> ) concentration and temperature.

Benchmark continuum and kinetic simulations of argon microplasmas in the direct current and microwave regimes

This study presents benchmark comparisons between continuum and kinetic simulations of argon microplasmas operating in the direct current and microwave regimes. Kinetic simulations using the particle-in-cell with Monte Carlo collisions (PIC-MCC) method and continuum simulations using the full-momentum equation for both ions and electrons are performed at various operating conditions in order to study the influence of product of pressure and gap size, pd (for a given gap size), influence of gap size (for a given value of pd) and operating frequency. It is shown that using the electron energy distribution function (EEDF) predicted by zero-dimensional Boltzmann solvers (such as BOLSIG+) in continuum simulations of direct current microplasmas leads to a significant under-prediction of plasma number densities with continuum simulations based on the Maxwellian EEDF performing better particularly for higher values of pd. The discrepancy between kinetic and continuum simulations is attributed to the presence of hot electrons created as a result of secondary emission and subsequent acceleration in the sheath. On the other hand, simulations performed for argon microwave microplasmas operating at 0.5 GHz, 0.8 GHz, 2 GHz and 4 GHz demonstrated that continuum simulations performed using the rate constants from BOLSIG+ showed excellent agreement with kinetic simulations for the plasma density profiles in spite of over-predicting the voltage/power required to achieve a given plasma density.

Trace determination of Hg together with As, Sb, Se by miniaturized optical emission spectrometry integrated with chemical vapor generation and capacitively coupled argon microwave miniplasma discharge

A miniaturized optical emission spectrometer (OES) with capacitively coupled argon microwave microplasma (μCMP) as and excitation source and chemical vapor generation (CVG) for sample introduction was constructed for the determination of trace Hg, As, Sb and Se. The applied method enabled simultaneous determination of hydride-forming elements (As, Sb, Se) and volatile Hg. Mercury cold vapor and the hydride volatile species of As, Sb and Se were generated when standard or sample solutions were separated from the liquid phase for transport to the capacitively coupled microwave microplasma and detection of their atomic emission. A univariate approach and the simplex optimization procedure were used to achieve optimized conditions and derive analytical figures of merit. The experimental concentration detection limits (LODs) for simultaneous determination, calculated as the concentration giving a signal equal to three times of the standard deviation of the blank (LOD, 3σblank criterion, peak height) were 3.0, 1.4, 1.5 and 3.8ngmL−1 for Hg, As, Sb and Se, respectively. The method was validated by the analysis of three Certified Reference Materials (NIST 2711, NRCC DOLT-2, NIST 1643e) of different matrix composition and by the standard addition technique. The method offers relatively good precision (RSD ranged from 5% to 8%) for microsampling (200μL) analysis. The measured of contents of elements in certified reference materials were in good agreement with the certified values (Hg 1.99–6.25μgg−1, As 16.6–105μgg−1, Sb 19.4–56.88μgg−1, Se 1.52–11.68μgg−1), according to the Student t-test, for a confidence level of 95%.

A novel, all-dielectric, microwave plasma generator towards development of plasma metamaterials

A proof of concept for a microwave microplasma generator that consists of a halved dielectric resonator is presented. The generator functions via leaking electric fields of the resonant modes — TE01δ and HEM12δ modes are explored. Computational results illustrate the electric fields, whereas the stability of resonance and coupling are studied experimentally. Finally, a working device is presented. This generator promises potentially wireless and low-loss operation. This device may find relevance in plasma metamaterials; each resonator may generate the plasma structures necessary to manipulate electromagnetic radiation. In particular, the all-dielectric nature of the generator will allow low-loss interaction with high-frequency (GHz–THz) waves.

Microwave air plasmas in capillaries at low pressure II. Experimental investigation

This work presents an experimental study of microwave (2.45 GHz excitation frequency) micro-plasmas, generated in dry air (N2 80%: O2 20%) within a small radius silica capillary (345 µm inner radius) at low pressure (300 Pa) and low powers (80–130 W). Experimental diagnostics are performed using optical emission spectroscopy calibrated in absolute intensity. Axial-resolved measurements (50 µm spatial resolution) of atomic transitions N(3p4S) → N(3s4P); O(3p5P) → O(3s5S) and molecular transitions N2(C,v′) → N2(B,v″); (B,v′) → (X,v″) allow us to obtain, as a function of the coupled power, the absolute densities of N(3p4S), O(3p5P), N2(C), N2(B) and (B), as well as the gas (rotational) temperature (700–1000 K), the vibrational temperature of N2(C,v) (7000–10 000 K) and the excitation temperatures of N2(C) and N2(B) (11 000 K). The analysis of the Hβ line-width gives an upper limiting value of 1013 cm−3 for the electron density; its axial variation (4 × 1011–6 × 1012 cm−3) being estimated by solving the wave electrodynamics equations for the present geometry, plasma length and electron–neutral collision frequency. The experimental results were compared with the results from a 0D model, presented in companion paper I [], which couples the system of rate balance equations for the dominant neutral and charged plasma species to the homogeneous two-term electron Boltzmann equation, taking the measured gas temperature and the estimated electron density as input parameters. Good qualitative agreement is found between the measurements and calculations of the local species densities for various powers and axial positions. The dissociation degree of oxygen is found above 10%. Moreover, both the measurements and calculations show evidence of the non-equilibrium behavior of low-temperature plasmas, with vibrational and excitation temperatures at least ten times higher than the gas temperature. These observations confirm that low-pressure microwave micro-plasmas exhibit high-reactivity, and the present study shows that these features are controlled by electron kinetics, as expected.

Operating modes of field emission assisted microplasmas in the microwave regime

The operating modes of microwave microplasma devices integrated with field emitting cathodes are studied using one-dimensional particle-in-cell with Monte Carlo collision (PIC-MCC) simulations. The PIC-MCC simulations predict operation in two modes—an α-mode characterized by a positive differential resistance with negligible influence of boundary processes and a γ-mode with significant field-induced electron emission. PIC-MCC results are presented for two representative 0.5 GHz argon microplasmas operating in the α and γ modes. The field emission-induced transition to γ-mode modifies the electron number density profiles in the sheath apart from leading to a higher contribution of conduction current in the sheath. The interpretation of electrical characteristics using time history of voltage and current demonstrates that the microplasma device impedance decreases as a result of the thinner sheath. It is also shown that the presence of field emitting cathodes leads to lower power requirements (about 64% of the case presented without field emission) to achieve a given plasma density.

Generation of surface-wave microwave microplasmas in hollow-core photonic crystal fiber based on a split-ring resonator.

We report on a new and highly compact scheme for the generation and sustainment of microwave-driven plasmas inside the core of an inhibited coupling Kagome hollow-core photonic crystal fiber. The microwave plasma generator consists of a split-ring resonator that efficiently couples the microwave field into the gas-filled fiber. This coupling induces the concomitant generation of a microwave surface wave at the fiber core surround and a stable plasma column confined in the fiber core. The scheme allowed the generation of several centimeters long argon microplasma columns with a very low excitation power threshold. This result represents an important step toward highly compact plasma lasers or plasma-based photonic components.

Experimental Investigation of Low Power Microwave Microplasma Source

The aim of this paper is to present a novel microwave microplasma source generated in di erent gases at atmospheric pressure. The design, rule of operation and experimental investigations of the new microwave microplasma source are described. The main advantage of the presented microwave microplasma source is its small size, simplicity, and low cost of construction and operation. The microplasma has a form of a small plasma jet of dimensions of a few mm, depending on the kind of gas, gas ow rate, and absorbed microwave power. Presented in this paper results of experimental investigations were obtained with an atmospheric pressure argon, krypton, nitrogen, and air microplasma, sustained by microwaves of standard frequency of 2.45 GHz. The absorbed microwave power was up to 70 W. The gas ow rate was from 2 to 25 l/min. The miniature size, simplicity of the source and stability of the microplasma allow to conclude that the presented new microwave microplasma source can nd practical applications in various elds.

Atmospheric pressure microwave microplasma microorganism deactivation

This paper is focused on the experimental investigations of microorganism decontamination by using low temperature Ar and Ar/O2 microwave microplasma. Microplasma in the form of a microflame was generated using a simple coaxial microwave microplasma source (MmPS). The MmPS was operated at standard microwave frequency of 2.45GHz. The electron density, microplasma temperatures and active species identification were determined on the way of Optical Emission Spectroscopy. The results of the spectroscopic measurements confirmed the MmPS usefulness in biomedical applications. The microplasma deactivation concerned two types of bacteria (Escherichia coli, Bacillus subtilis) and one fungus (Aspergillus niger). The investigations involved influence of the O2 concentration, absorbed microwave power, microplasma treatment time and microplasma distance from the treated sample on the microorganism deactivation efficiency. All reported results were obtained for Ar and Ar/O2 microplasma with gas flow rates of single l/min and O2 admixture not exceeding 2%. The absorbed microwave power was up to 50W. The sample treatment time was up to 10s.

Atmospheric pressure low-power microwave microplasma source for deactivation of microorganisms

This work was aimed at experimental investigations of deactivation of different types of microorganisms by using atmospheric pressure low-temperature microwave microplasma source (MmPS). The MmPS was operated at standard microwave frequency of 2.45 GHz. Its main advantages are simple and cheap construction, portability and possibility of penetrating into small cavities. The microplasma deactivation concerned two types of bacteria ( Escherichia coli , Bacillus subtilis ) and one fungus ( Aspergillus niger ). The quality as well as quantity tests were performed. The influence of the microorganism type, oxygen concentration, absorbed microwave power, microplasma treatment time and MmPS distance from the treated sample on the microorganism deactivation efficiency was investigated. All experiments were performed for Ar microplasma and Ar/O 2 microplasma with up to 3% of O 2 . Absorbed microwave power was up to 50 W. The Ar flow rate was up to 10 L/min. The sample treatment time was up to 10 s.

Spectroscopic investigations of microwave microplasmas in various gases at atmospheric pressure

In this paper results of the experimental investigations of a coaxial microwave (2.45 GHz) microplasma source (MMS) with graphite or tungsten inner conductor operated in Ar, N2 and Ar/C2H2 mixture at atmospheric pressure are presented. The microwave power absorbed by the microplasmas and the intensity of UV-C emission from the microplasmas were measured. Using optical emission spectroscopy, the electron number density in Ar microplasma, and rotational and vibrational temperatures in N2 and Ar/C2H2 microplasmas were determined. All experiments were performed with a gas flow rate from 0.3 to 8 l/min and absorbed microwave power from 5 to 300 W. The simplicity of the MMS, stability of its operation with atmospheric pressure gases, and parameters of the microplasmas allow concluding that the MMS can be used in various applications.

Microwave microplasma sources based on microstrip-like transmission lines

In this paper, we study two microwave sources based on a planar transmission line configuration, corresponding to linear resonators. In both sources, micro-plasmas are produced within the 50–200 \(\mu \)m gap created between two metal electrodes placed at the open end of a microstrip-like transmission line. The study of the sources follows a complementary approach that uses simulation and experiment. Simulations analyze the electromagnetic behavior of the sources, using the commercial tool CST Microwave Studio®, and characterize the plasmas produced, using a fluid-type code to describe the dynamics of charged particles. Experimentally, the return loss of the sources (hence their quality factors) is measured without and with plasma. Plasma diagnostics (in air and in argon), based on optical emission spectroscopy measurements, enable to obtain the typical plasma temperatures and the electron density (using Stark broadening measurements of the H\(_{\beta}\) line-emission profile). Results reveal that the sources have similar quality factors (~15–20), yielding high-density (~1014 cm\(^{-3})\), low-power (~10–50 W), non-equilibrium micro-plasmas (with rotational temperatures of ~950–1400 K in air and ~550–630 K in argon, vibrational temperatures of ~5200–5800 K in air and excitation temperatures of ~5800 K in argon), over volumes of ~10-4–10-3 cm3.

Fluid modeling of a microwave micro-plasma at atmospheric pressure

This paper presents the modeling of an argon micro-plasma produced by microwaves (2.45 GHz) at atmospheric pressure. The study uses a one-dimensional stationary fluid-type code that solves the transport equations for electrons, positive ions Ar + and Ar + 2 , and the electron mean energy, together with Poisson's equation for the space-charge electrostatic field, Maxwell's equations for the electromagnetic excitation field and the gas thermal energy equation. The model uses a simple kinetic scheme for Ar that includes the ground state, an excited state representing the lumped 4s levels, and two ionization states associated to the atomic and the molecular ions. The ions are assumed to be in thermal equilibrium with the neutral gas, having the same temperature profile.

A simple collisional–radiative model for low-temperature argon discharges with pressure ranging from 1 Pa to atmospheric pressure: kinetics of Paschen 1s and 2p levels

A simple collisional–radiative model for the Paschen 1s and 2p levels is proposed for low-temperature argon discharges. This model can predict the population distribution of 1s and 2p levels over a wide discharge pressure range 1–105 Pa and ionization ratio range 10−6–10−3. The modelling results are found to be in good agreement with observed optical emissions from several different types of argon discharges at 1, 100 and 105 Pa. By using the model, the dominant kinetic processes of 1s and 2p levels are investigated for an electron beam plasma, an inductively coupled plasma, a capacitively coupled plasma and a microwave microplasma. A kinetic diagram is given, which can be used to identify the kinetic state of 1s and 2p levels in many low-temperature argon discharges reported in the literature. This model is also useful for obtaining discharge parameters from optical emissions in low-temperature argon discharges.

Fluid modeling of a microwave micro-plasma at atmospheric pressure

This paper presents the modeling of an argon micro-plasma produced by microwaves (2.45 GHz) at atmospheric pressure. The study uses a one-dimensional stationary fluid-type code that solves the transport equations for electrons, positive ions Ar+ and Ar+2, and the electron mean energy, together with Poisson's equation for the space-charge electrostatic field, Maxwell's equations for the electromagnetic excitation field and the gas thermal energy equation. The model uses a simple kinetic scheme for Ar that includes the ground state, an excited state representing the lumped 4s levels, and two ionization states associated to the atomic and the molecular ions. The ions are assumed to be in thermal equilibrium with the neutral gas, having the same temperature profile.

Design of a Microwave Microplasma Source at Atmospheric Pressure

This paper studies two linear resonator sources, which use a continuous 2.45-GHz microwave excitation to produce stable microplasmas, in air and in argon, at atmospheric pressure. The discharges are produced and sustained within the 50-200-mum gap created between two metal electrodes with either 6 or 14 mm in length. Particular attention is given to the design and optimization of the sources (in terms of frequency tuning and power coupling), following a complementary approach based on simulations and experiments. Optical-emission-spectroscopy diagnostics allow one to deduce the rotational, vibrational, and excitation gas temperatures and the electron density (using Stark broadening measurements of the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">beta</sub> line-emission profile).

The development of microplasmas for spectrochemical analysis

Miniaturized microwave, high-frequency, and dc-powered microplasmas are discussed, with emphasis on the state-of-the-art and development trends. Specific atomic emission sources discussed include the microstrip microwave plasma operated in argon and helium at ca 10-30 W and below 1 L min(-1) gas at atmospheric pressure, the capacitively coupled microplasma, operated at 13.56 MHz, 5-25 W, and 17-150 mL min(-1) helium, the miniaturized inductively coupled plasma operated at several watts and reduced pressure, and dc glow-discharge plasmas on a chip, including a barrier-layer discharge as atom reservoir for atomic absorption spectrometry. Diagnostics for these sources are discussed and some of their figures of merit are compared with those of conventional sources. Current possibilities for introduction of gaseous samples are reported and scope for further development and outlook are both discussed.